Last reviewed and updated: 2026-09-24
A warm or hot patch on the floor of a slab-on-grade home usually means a hot water supply line running under the concrete is leaking, and the escaping hot water is warming the slab above it. It is one of the most specific symptoms in plumbing. Confirm it by isolating the water heater supply and watching the meter: if the meter stops when the hot side is closed, the leak is on the hot run.
A warm patch is the classic hot-side slab leak sign. Thermal imaging maps it, and the bill it produces is covered on the high water bill page.
What to check first
- Rule out the obvious. Sunlight through a window, a heating duct beneath the floor, a radiant heating system, a nearby appliance or an electrical fault can all warm a floor. Check whether the patch is warm at night with the curtains closed.
- Map the patch. Note where it is and how large. A leak plume grows slowly as more concrete heats up.
- Test the hot side. Close the cold supply valve into the water heater. Watch the meter. If it keeps moving, the leak is on the hot run, consistent with what you are feeling.
- Listen at the patch. Put an ear to the floor there with the house quiet. A pressurized slab leak is often audible.
- Look for the supporting signs. A water bill climbing steadily, damp carpet, cupping hardwood, cracked tile, a musty smell, or lower hot water pressure.
Why it happens in Marietta homes
Homes built here between roughly the 1960s and 1980s commonly had copper supply lines run under the slab. Copper in direct contact with concrete and soil corrodes from the outside over decades, and hot lines fail first, heat accelerates corrosion and the pipe expands and contracts against the concrete with every use. Georgia's expansive red clay adds seasonal ground movement on top of that.
What happens if it is left
| Timescale | What typically develops |
|---|---|
| Days | Water bill rising; the warm area spreading as more concrete heats |
| Weeks | Flooring damage: cupping hardwood, lifting laminate, cracked or loose tile |
| Weeks to months | Moisture through the slab feeding mold in flooring, skirting and cabinetry |
| Months | Soil beneath the slab washing or softening, which can move the foundation |
| Any point | Insurance characterising the damage as gradual rather than sudden, which affects the claim |
You can stop the loss today. Closing the cold inlet valve to the water heater between uses stops the hot side leaking while you arrange detection. You lose hot water, not the house.
Frequently asked questions
What causes a warm spot on a floor?
In a slab home, most often a hot water supply line leaking under the concrete, warming the slab above. Other explanations worth ruling out are sunlight through a window, a heating duct under the floor, radiant heating, a nearby appliance, or an electrical fault.
How do I confirm it is a hot water slab leak?
Close the cold supply valve into the water heater and watch the water meter. If the meter keeps moving with the hot side isolated, the leak is on the hot run, which matches a warm patch on the floor.
Will a slab leak get worse?
Yes. It runs continuously, so the bill accrues daily, the warm area spreads, flooring begins to cup or crack, and moisture through the slab feeds mold. Over longer periods water can soften or wash out the soil supporting the foundation.
Can I stop it myself in the meantime?
Closing the cold inlet valve to the water heater stops the hot side leaking. You lose hot water until it is repaired, but the loss and the damage stop while you arrange detection.
Why do hot water lines fail before cold ones?
Heat accelerates corrosion, and the pipe expands and contracts against the surrounding concrete every time hot water runs through it. Both mechanisms wear the pipe wall faster than on the cold side.